The present invention relates generally to the technology of transmission via optical fibers and more particularly to a ring network using a dual optical data bus.
The ever-increasing use of public and private networks to transport data has for some years now given rise to an enormous demand for bandwidth, for exchanging ever-increasing quantities of information and setting up all forms of communication, from exchanging simple text and picture files to the considerable expansion of electronic mail (E-mail) and all types of business massaging, via voice transport, including conventional telephony using the time division multiplexing (TDM) mode and protocols, and even the transport of voice in packet mode using Voice over IP (VoIP) exchange protocols, video distribution and, of course, all the applications resulting from the considerable expansion of the Internet and especially its universally used main application, the World Wide Web (WWW).
To face up to this demand for bandwidth, those responsible for providing the networks soon had to have recourse to transporting the signals carrying the information in optical form, so as to benefit firmly from the low cost of the fibers themselves and secondly from the very high bit rates that can be achieved, despite transmission distances measured in kilometers, or even tens or hundreds of kilometers, without having to regenerate the signal, attenuation being very low, especially in mono mode fibers, compared to the attenuation resulting from electrical transmission over copper cables, for example. Moreover, optical transmission avoids all the problems that are associated with electromagnetic interference and which necessitate costly protection circuits and can lead to frequent transmission errors.
The first use of optical fibers was essentially in point-to-point connections. Thus, the transport signals are converted into light signals that are transmitted between two nodes of a network and are immediately converted back into electrical signals on reception, in order to be processed in the receiver node which, after analyzing the information received, must either relay the information to another node of the network or use the data locally. In the former case further electrical-to-optical conversion is needed, of course, even though the payload information transported has not been modified and only the final destination has had to be examined.
This mode of operation is used in Synchronous Optical NETwork (SONET) and Synchronous Digital Hierarchy (SDH) rings in particular, which are respectively the standard for North America and for Europe, and are for the most part compatible. In particular, the transmission bit rates are standardized, and the higher bit rates that are most widely used are 2.48 Gb/c (SONET OC-48 and SD STEM-16), 10 Gb/c (SONET OC-192 and SD STEM-64) and even 40 Gb/c (SONET OC-768 and SDH STM-256). It is to be noted that, as shown in FIG. 1, these rings in fact consist of a double ring of optical fibers 100, one of which is an inactive protection channel and is used only in the event of a break in the active fiber, after fast automatic protection switching (APS), taking less than 50 milliseconds, has taken place to assure the continuity of traffic that constitutes an essential quality criterion for networks of this kind transporting enormous quantities of data. The nodes of these networks are generally add/drop multiplexers (ADM) 110 which provide local access to a portion of the data stream without affecting the remainder of the traffic, although necessitating at each node optical-electrical-optical (O-E-O) conversion of the traffic. SONET/SDH rings have been very successful and are still very widely used, because of the APS system referred to above, and their inherent capability of adapting to the ever higher transmission speeds required by the extremely rapid expansion of the amounts of data exchanged characteristic of the last several years.
This first applications of optical fibers nevertheless soon proved to be insufficient. Although the fiber itself is of relatively low cost, its deployment can be very labor-intensive and prove extremely costly. Rather than deploy more optical fibers when the capacity of an installed network becomes insufficient, the solution has been adopted of making better use of the fibers already in place. By transmitting different frequencies in the same fiber, the wavelength division multiplexing (WDM) technique multiplies the number of completely independent transmission channels on the same physical fiber. In other words, transmitting light rays of different “colors” multiplies the bandwidth of a single fiber commensurately. The dense WDM (DWDM) technique, which soon succeeded the WDM technique, can multiplex 80 or even more channels.
Although the above techniques provided an effective response to the enormous demand for bandwidth, this led to the development of an optical data transport layer that itself gave rise to a few problems. The essential reason for this was that, in the current state of the art, the signals and the data transported are still processed essentially in the electrical domain. Accordingly, optical/electrical conversion is essential each time that the transported data has to be examined. In particular, in systems transporting information in packet mode, it is usually necessary to consult the packet headers at each network node to determine the next hop (i.e. the next destination node). This is the case with the Internet Protocol (IP) in particular, which is obviously in very widespread use, and which operates in the connectionless mode, in contrast to other protocols for which a path must be set up beforehand, by means of appropriate signaling, before the exchange of data can take place. This is the case with telephony and with the TDM modes of transport previously mentioned.
Thus although the optical transport of signals for exchanging data is intrinsically of relatively low cost, electrical/optical and optical/electrical conversion remain costly. In particular, light is almost always injected into an optical fiber from lasers, which have to be more sophisticated to mix numerous wavelengths in the same fiber (as in the DWDM technique). This is because the various “colors” or wavelengths used are much closer together, of course, and there is the risk that their emission spectra may overlap, making it impossible to distinguish them on reception unless the lasers used are capable of emitting in a very narrow frequency band, which makes them more difficult to produce and therefore more costly.
The document EP 1.128.585 describes a multilevel optical network. The lowest level comprises a ring and stations coupled to the ring by access nodes that do not necessitate optical/electrical or electrical/optical conversion. The ring comprises two optical fibers carrying uplink information and downlink information, respectively. Each node is connected to a plurality of stations by a star network. Each access network comprises:                two passive optical couplers connected to respective fibers,        a demultiplexer for downlink signals connected to a first coupler, and        a multiplexer for uplink signals connected to the second coupler.Each station is connected to the access node by a single fiber that is specific to it. Onto this fiber are multiplexed a wavelength carrying an uplink signal and a different wavelength carrying a downlink signal. Each station communicates with the remainder of the node by means of two wavelengths that are specific to the station.        